1. Field of the Invention
The present invention relates to obtaining seismic data or information.
2. The Prior Art
For the seismic study of subterranean formations, a perturbation is produced in these formations, for instance by means of an underground explosion or of a powerful vibrator, and the seismic signals consequent on this perturbation are recorded after they have been picked up by detectors distributed over the ground in appropriate configurations.
Recording was at first effected in analogue form; the recorded signal varied as the signal picked up by the detectors. Seismic signals soon came to be recorded in digital form; the recorded signals now represent a sufficiently great number of measurements of the signals picked up by the detectors to constitute a satisfactory representation of them.
The main difficulty in recording is that the seismic signals have a very great dynamic range, extending over 140 decibels. In the former technology efforts were made continually to improve the fidelity of recording. This has been made possible firstly by the recording of signals in digital form instead of analogue signals, and secondly by the fact that the measurement giving these digital signals has been made more and more precise. We have thus reached on the one hand a field of measurement in which the scale is extremely fine, that is to say it has a large number of points, and on the other hand floating point digitization; in this, each seismic signal sample is amplified, before being measured, with a variable gain so determined that its comparison with the very fine scale of measurement reliably provides a maximum number of significant figures.